[torstesu]

In my opinion, hardware buttons should be limited to only non-critical operations or operations with limited usage frequency (which may be an argument to why the button may be unnecessary in the first place).

A button such as the back-button on the android, is used to navigate in almost all applications. With the amount of "load cycles" the back-button experiences, it is bound to either fail or start operating inadequately [1]. If or when this happens, the phone becomes rather unusable as it is nearly impossible to navigate in applications that do not have any built in user interface options to perform the same action.

[1] I do not hold evidence for this to be true in general and my hypothesis is only backed by personal experience and the notion of failure rate in a load frequency perspective.

[mikeash]

Given how long keyboards last under far heavier and harsher loads than any phone button experiences, it's clearly not a given that a hardware button must start having problem during the lifetime of the product. But it does seem clear that manufacturers aren't interested in putting the same durability on phones as they are on keyboards, as I've seen these problems with iPhone sleep and home buttons.